Polyurethane foams, formed by the reaction of a polyisocyanate with a polyhydroxyl-containing compound in the presence of a suitable catalyst, are widely accepted as padding materials for cushions in furniture, automobiles and the like. Polyurethane foams are also used in sponges and for other uses that require high liquid absorption properties, such as specialty packaging and personal care and hygiene items, including highly absorbent diapers. Polyisocyanurate foams, made with a different catalyst to increase cross-linking relative to polyurethane foams, are often used as insulation in the building and construction industry.
The components used to make polyurethane foams have often been modified or added to in attempts to improve one or more properties of the resultant foam. For example, polymer polyols, made by the polymerization of monomers in the presence of a base polyol, are often used to supplement or in place of the polyhydroxyl-containing component to enhance polyurethane foam properties. Various other materials are added to the foam formulation such as antioxidants, stabilizers and flame retardants. Flame retardants inhibit the burning of a polyurethane foam should it ignite. Typical flame retardant additives include, but are not limited to, antimony trioxide, phosphate ester plasticizers and halogenated compounds.
A common problem with additives or fillers to polyurethane foam formulations is that the distribution of the additive throughout the formulation may not be uniform which results in isolated areas of the foam that are defective or that at least do not posses the benefits desired throughout the foam. Another typical disadvantage is that other foam properties, such as load bearings properties, tear and tensile strengths, etc., may be degraded simply by the physical presence of some additives. It would be advantageous if fire retardant polyurethane foams could be produced without the extra addition of a separate material and/or the degradation of final foam properties.
Interest in polyurethane foams with enhanced ignition resistance has dramatically increased throughout the world since the introduction of new flammability standards for foams in Great Britain. However, it is also generally accepted that foams having flame retardants which contain phosphorus and halogens, as noted above, have limitations in meeting current fire retardancy requirements, and more particularly, the recent British Standard 5852, Part 2, Ignition Source 5 specifications. The technologies now used to produce foams meeting these new combustion requirements involve the addition of substantial amounts of solid flame retardant, predominantly melamine, but also aluminum hydrate or exfoliated graphite with increased level of standard liquid flame retardant to foam formulation.
As noted, the addition of solid flame retardants, such as melamine, to provide adequate combustion modification negatively influences some of the major foam processing parameters. For example, the viscosity of the polyol-filter dispersion is high; the cell opening is non-uniform, resulting in a significant top-to-bottom density gradient, and large amounts of undesirable chlorofluorocarbon blowing agents are required to lower the density.
U.S. Pat. Nos. 3,655,553; 3,953,393; and 4,214,055 describe polymer polyols made by homo- or copolymerizing vinyl chloride or vinylidene chloride in a polyol. These materials are claimed to produce polyurethane foams with improved fire retardant properties. However, there are problems with these materials. Polymers of vinyl and vinylidene chloride are thermally unstable. Thermal decomposition proceeds with the evolution of hydrogen chloride. This results in products which are discolored and have high acid numbers. The high acidity causes urethane catalysis problems.
The preparation of urethane foams having reduced flammability from polymer polyols made by copolymerizing bis(.beta.-chloroethyl)vinyl phosphonate in a polyol is described in U.S. Pat. No. 3,925,506. Vinyl-phosphonates are very unreactive monomers and do not incorporate into the polyol in high conversion. None of these patents address the difficulty of meeting the increased fire retardancy standards, discussed above.